1. Field of the Invention
Embodiments of the present invention relate generally to time and frequency alignment systems operating over communications networks and, more specifically, to methods and apparatus for precision time transfer wherein the inherent asymmetry error introduced in networks is estimated and compensated for.
2. Description of the Related Art
It has been recognized that synchronizing network elements in communications networks to a high level of precision enables the provision of advanced services. In fact, time and frequency alignment are essential to certain types of systems. For example, accurate time alignment is required by cellular networks, services carried out in real time over a network, and network elements that use packet-based signal formats for multiplexing, transmission, and switching. Similarly, frequency alignment is required in time-division multiplexing (TDM) and media streaming systems that require fixed video or audio sample rates across multiple clients. Modern mobility services are demanding more and more bandwidth and lower and lower latency, placing increasing demands for tight time coordination in the wireless transport (radio access networks). Another example is the demand for much higher time coordination of transactions in high speed financial trading. To gain some perspective into the increasing demand on precision, whereas legacy time distribution targets at the more stringent end are of the order of sub-milliseconds, emerging application and services targets are now of the order of sub-microseconds.
The distribution of time over packet networks is now ubiquitous. The dominant method is the use of the Network Timing Protocol (NTP) for support of general timing applications in general computing applications. However, these implementations, based on existing standards and conventions are suitable for time alignments of the order of (several) milliseconds. Over the last decade, a new protocol, Precision Timing Protocol (PTP) has emerged supported by industry standards (IEEE 1588v2, ITU G.827x series). The key differentiator between NTP and PTP is that the new levels of precision that can be obtained with PTP will support the needs of a variety of new applications and services. Both PTP and NTP are protocols for exchanging time-stamps associated with time-of-arrival and time-of-departure of designated packets and are thus, in principle if not practice, capable of similar performance levels.
In both cases the single, dominant, source of time error, error that cannot be corrected by the protocol, is asymmetry. Asymmetry as considered here is the difference in transit delay of the designated event packets in the two directions between the communicating clocks. Whereas packet delay variation is an expected phenomenon in packet networks and does contribute to asymmetry, there is an underlying asymmetry component that is entirely independent of the network loading and depends substantively only on the path between the clocks. The path, as considered here, includes all transmission links, including multiplexers and signal-format converters and transmission media, and intermediate network elements, such as switches and routers, between the communicating clocks.
One approach known in the art that provides both time and frequency alignment involves computing an aligned time signal based on a master timing signal from a primary reference clock, such as a global navigation satellite systems (GNSS) satellite timing signal, which is held in precise alignment with a global clock reference. The most common GNSS in use is the Global Positioning Satellite (GPS) system. This is depicted in FIG. 1. The two clocks, CLOCK-1 110 and CLOCK-2 120 both receive timing signals 130 from the GNSS system 150. By aligning themselves to the GNSS timescale, the two clocks are, albeit indirectly, aligned to each other. Using GPS signals or other master timing signals at each network element to achieve time or frequency alignment requires each network element to be able to receive satellite time signals from GPS satellites. There are many situations where visibility of GPS satellites may be compromised, interfered with, or interrupted. It is generally accepted that whereas GPS is a reliable and robust system, in many installations the visibility of a sufficient number of satellites simultaneously may not be available continuously.
Packet-based schemes such as PTP and NTP are discussed next. One of the principal drawbacks of such schemes for transferring time is the potential asymmetry of the transit delay between the two clocks. Such asymmetry cannot be determined or estimated using PTP/NTP itself. The negative impact of such network impairments can be mitigated by combining PTP/NTP packet-based methods with other, complementary, GNSS-based schemes (e.g. GPS).
Packet-based network synchronization methods such as Precision Time Protocol (PTP) and Network Time Protocol (NTP) transfer time and frequency references using packets containing time stamps that identify the times of departure/arrival of packets. These protocols can be used to distribute timing and frequency alignment throughout a network in a point-to-point fashion similar to the way that TDM networks distribute frequency alignment, as described above. For specificity the discussion here considers PTP though essentially identical statements apply in the case of NTP and all other packet-based time transfer methods.
A PTP “grandmaster” can transfer time to a network element coupled to it, a “slave” instantiated on that network element can reconstitute the time, and then a “master” connected to the slave in the same network element can transfer time to a subsequent network element in this point-by-point fashion. PTP and NTP can also operate in a mode where the “slave” clock in a network element can communicate directly with the “master” clock system for timing purposes. In all cases, the accuracy of two-way time-transfer protocols is adversely affected by asymmetry introduced by the communications network connecting the two network elements, including asymmetry in the physical medium, asymmetry in the construction of the forward and reverse paths in the network elements, and other sources. PTP and NTP assume that transit delays between master and slave clocks are symmetric, i.e., the transfer packet delay from a master clock to a slave clock is equal to the transfer packet delay from the slave clock to the master clock. But because forward and reverse physical paths are often different in coupled network elements, they are typically not symmetric. The goal of packet-based methods is to establish the “offset from master” (OFM) of the slave. That is, if we denote the master time by TM and the slave time by TS, then the goal is to establish ε where TS=TM+ε.
The premise of time-stamped packet exchange as used in PTP (and NTP as well) is illustrated in FIG. 2 (prior art). Traditional packet-based two-way time transfer methods follow the event diagram shown in FIG. 2. The terminology used here is that from PTP but the same principles apply to all methods and protocols. Referring to FIG. 2, the sequence of events and important items of information associated with an exchange of packets between master 210 and slave 220 are:
Event A 230: Packet is transmitted by Master and time-of-departure is t1.
Event B 232: Packet arrives at Slave that measures the time-of-arrival as τ2; assuming that the slave time offset from master is ε, the actual time-of-arrival with respect to the master timescale is t2=τ2−ε.
Event C 234: Packet is transmitted by Slave that notes the time-of-departure is τ3; assuming that the slave time offset from master is ε, the actual time-of-departure with respect to the master timescale is t3=τ3−ε.
Event D 236: Packet arrives at Master that measures time-of-arrival as t4.
Such a two-way exchange of packets can provide information suitable for allowing the slave to align in time with the master (assuming that both sides have knowledge of the time-stamps). There are four measured values that can be communicated between the Master and Slave, namely, (t1, τ2, τ3, t4). Denoting by ΔMS and ΔSM the transit delays between the Master and Slave and vice versa, the following equations can be established:t4=τ3−ε+ΔSM (from an S-to-M packet)t1=τ2−ε−ΔMS (from a M-to-S packet)  (Eq. 1)
The round-trip delay, RTD, is estimated simply as:RTD=(t4−τ3)+(τ2−t1)  (Eq. 2)
The two quantities in parentheses in the right-hand side of (Eq. 2) comprise the reverse and forward offset measurements, respectively.
Note that there are two equations with three unknowns (ε, ΔMS, ΔSM) so it is common in conventional PTP methods to assume reciprocity of transit delay between the two devices, thereby reducing the number of unknowns to 2 and therefore computing ε, the slave time offset from master. This assumption implies that there is an inherent error in the time transfer that is related to the asymmetry of the transit delay in the two directions. Specifically, the error in time transfer will have an error that is nominally
                              ɛ          A                =                                            Δ              MS                        -                          Δ              SM                                2                                    (                  Eq          .                                          ⁢          3                )            The asymmetry in transit delay of timing packets between slave and master provides a statistical bound to the accuracy of time transfer that can be guaranteed. For example, packet-based methods like PTP and NTP use separate fiber strands or fiber wavelengths for carrying the signal in the two directions (S-to-M and M-to-S). Consequently the difference in the length of the fiber strands in each direction introduces an asymmetry in transit delay. Since the velocity of light in the fiber is wavelength dependent, using different wavelengths in the two directions introduces an asymmetry in transit delay. These are in addition to asymmetry that may result from the behavior of intervening network elements. The example of fiber transport just cited is one of a number of different transport options supported the effective physical layer of the protocol stack. The network path may be over any network transport including a variety of physical layers transports including but not limited to:                1. Native Physical Layer Ethernet        2. SONET/SDH (Synchronous Optical Network)        3. Optical Transport Network (OTN)        4. Microwave radio        5. Millimeter wave radio        6. Non-Line-of-Sight Radio        7. xPON (different flavors of Passive Optical Networking)        8. DOCSIS (standard applicable to transmission over cable-TV networks)        9. Infiniband (standard for intra-computer-system communication)        
The transport technology introduces some transit delay impairment in the form of delay variation. In addition, since the transmission path is distinct in the two directions, transit delay asymmetry can be introduced. Prior art methods for addressing the asymmetry include calibration of the transit delays in the two directions. The calibration step is generally performed when there is no information traffic such as during the equipment deployment phase or during intervals where the network is taken out of service for maintenance purposes or if the transmission medium is dedicated to timing with no information traffic present. Other methods in the prior art include the use of burst transmission employing the same link in the two directions as described in U.S. Pat. No. 8,594,134, entitled “Precision Time Transfer over Optical Fiber,” and the use of multiple wavelengths as described in U.S. Pat. No. 8,600,239, entitled “Precise Clock Synchronization over Optical Fiber.”
The asymmetry introduced by the transport layer is depicted in FIG. 3. For simplicity, the asymmetry is shown with delay greater in the reverse path (from slave to master).
Thus, the ability of PTP and NTP to accurately transfer time between network elements in a packet network is limited. Specifically, the asymmetry in transit delay of timing packets between slave and master clocks provides a bound to the accuracy of time transfer.